1. Field of the Invention
The present invention relates to the encoding of computer-generated images, and more particularly to a unified coding method for arrays of data, independent of the function of the data. Accordingly, arrays of position data, rotations, and normals are encoded consistently.
2. Discussion of the Background
The phrase "computer-generated images" encompasses an expanding area of video technology. Originally the term was often equated with simple text images or 2D images; however, the phrase now encompasses any type of digitally encoded video stream. The Motion Pictures Expert Group (MPEG) was formed to investigate the technologies required for the encoding and decoding of image streams. The resulting standard (now referred to as "MPEG-1") has served as a basis for two additional MPEG standards: MPEG-2 and MPEG-4. MPEG-4 is a standard that is "in progress" and forms the basis for the present invention. The final committee drafts are ISO/IEC FCD 14496-1 MPEG-4 Systems and -2 MPEG-4 Visual, the contents of the final committee drafts are incorporated herein by reference.
The draft standard departs from the single stream-based model of video and changes the focus to a series of streams that act in concert. One portion of the standard is the Binary Format for Scenes (also known as "BIFS"). This format allows the description of 3D objects and their motion and provides the ability for greater interaction with that portion of the video stream. The draft standard for MPEG-4 BIFS proposes to encode multiple data fields by a linear quantization process, which does not account for the correlation within those multiple fields. Thus, MPEG-4 BIFS does not describe predictive encoding of arrays.
Another proposed graphics format is the Compressed Binary Format for Virtual Reality Markup Language (hereinafter "CBF VRML") that includes an encoding method that is applicable only to a few fields of a few nodes and for a limited subset of data types. The fourth and fifth drafts of the CBF VRML standard, dated Aug. 22, 1997 and Oct. 15, 1997, respectively, are available from www.vrml.org and are incorporated herein by reference. Likewise, the Revised Requirements Statement for CBF VRML97, dated Apr. 3, 1998, is also incorporated herein by reference.
Yet another graphics format is the Tag Image File format (TIFF). The contents of version 6.0 of the TIFF specification are incorporated herein by reference. Each entry in a TIFF file is stored as four parts: (1) a tag, (2) a type identifier, (3) a count, and (4) one or more values. Page 16 of the version 6.0 specification states that "all fields are actually one-dimensional arrays, even though most fields contain only a single value." Moreover, although page 16 also states that there may be multiple images per file, the specification is designed for independently coded 2D images rather than streaming 3D images.